This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-107148, filed Apr. 5, 2001, No. 2001-129035, filed Apr. 26, 2001; and No. 2002-013333, filed Jan. 22, 2002, the entire contents of all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel assembly used in a nuclear reactor, and a thimble screw of the fuel assembly.
2. Description of the Related Art
An example of a nuclear reactor currently widely used for power generation includes a pressurized water reactor (to be referred to as xe2x80x9cPWRxe2x80x9d hereinafter). A fuel assembly used by the PWR is generally a canless fuel assembly with no wrapper tube. The structure of the canless fuel assembly will be briefly described. Top and bottom nozzles each having a plurality of coolant flow holes are connected to each other with a plurality of control rod guide tubes extending parallel to each other.
More specifically, the upper ends of the control rod guide tubes, i.e., so-called guide thimbles, are mechanically connected to the top nozzle, and the lower ends thereof are also mechanically connected to the bottom nozzle. These guide thimbles respectively accept the thin elongated control rods of a control rod cluster. Depending on the loading position of the fuel assembly in the core, the guide thimbles do not accept the control rods as they are not located at corresponding positions. In this case, the guide thimbles accept non fuel bearing components (NFBC) such as thimble plugs or burnable poisons. A plurality of grids are mounted on the guide thimbles. The fuel rods are accepted in the lattice openings and are elastically supported there.
Of the structure of the fuel assembly briefly described above, the structure of the connecting portion that connects the guide thimbles and bottom nozzle will be described in more detail with reference to the accompanying drawings.
FIG. 1 is an elevation schematically showing the structure of a fuel assembly applied to a PWR.
FIG. 2A is a sectional elevation showing part of the lower structure of this fuel assembly, and FIG. 2B is a bottom view of the same.
As shown in FIG. 1, a fuel assembly 1 has top and bottom nozzles 3 and 4 at upper and lower ends of elongated guide thimbles 2, and a top grid 5, middle grids 6, and bottom grid 7 fixed to the guide thimbles 2 in the longitudinal direction. Each of the top and bottom grids 5 and 7 is formed from a large number of lattice frames using thin plates, and holds fuel rods 8.
The top nozzle 3 is a bottomed box-like structure with a substantially square horizontal section. The top nozzle 3 has a plurality of coolant flow holes and guide thimble mounting holes in its end plate corresponding to the bottom plate. In addition, a hold down spring 9 is attached to the upper portion of the top nozzle 3. The bottom nozzle 4 has a top or end plate with a substantially square shape when seen from above, where a plurality of coolant flow holes and guide thimble mounting holes are formed. Legs 10 are respectively integrally formed to project from the four corners of the lower surface of the end plate.
The top and bottom nozzles 3 and 4 are connected to the upper and lower ends of the plurality of hollow tube-like guide thimbles 2 by utilizing the mounting holes described above.
Referring to FIGS. 2A and 2B, the lower end of each hollow tube-like guide thimble 2 is welded to a thimble end plug 12, and is fixed to the bottom nozzle 4 with a thimble screw 14 through an insert 13. One top grid 5 and seven middle grids 6 are mounted on the guide thimbles 2 at intervals, and the bottom grid 7 is mounted on the guide thimbles 2 through its connecting structure. It should be understood that the number of middle grids 6 can be appropriately changed.
The bottom grid 7 is fixed to the upper portion of the insert 13. A drain hole 15 extends through the thimble screw 14 in the axial direction, and a rotation preventive pin 17 for preventing loosening of the thimble screw 14 is provided to a seat 16 of the drain hole 15. The drain hole 15 allows the coolant in use to flow in the core in a direction P shown in FIG. 2A.
Furthermore, the seat 16 has a spot facing hole 18 communicating with the lower portion of the drain hole 15 and reaching the bottom surface of the seat 16. The rotation preventive pin 17 does not interfere with the flow of the coolant flowing into the drain hole 15 in the direction P.
The fuel rods 8 are inserted in and supported by the aligned lattice openings of the upper, middle, and bottom grids 5, 6, and 7 one by one, thus forming the fuel assembly 1.
With this structure, the drain holes 15 of the thimble screws 14 guide the coolant into the guide thimbles 2 in the core, and the introduced coolant cools the non fuel bearing components mounted in the guide thimbles 2. The drain holes 15 also serve as holes for sending the inner coolant to the outside.
During a scram mode of the nuclear reactor, the control rods are urgently inserted in the guide thimbles 2 by free fall. The drain holes 15 also serve as a restrictor for limiting the outlow velocity of the inner coolant so the fall impact is moderated. In other words, to assure the cooling function described above, the larger the diameter d of the drain hole 15 of the thimble screw 14, the better. To moderate the fall impact produced when the control rods fall, the smaller the diameter d, the better, which is contradictory.
During the scram mode of the nuclear reactor, when the control rods are urgently inserted in the guide thimbles 2 by free fall, an excessively large impact occurs to the top nozzle 3. For this reason, the guide thimbles 2 respectively have thin tube-like dashpots 20. The dashpots 20 reduce the velocity of the control rods falling in the guide thimbles 2, thereby moderating the excessively large impact acting on the top nozzle 3.
According to an example of the fuel assembly 1 with such dashpots 20, as shown in FIG. 3, a dashpot 20 with a length of 0.16 L to 0.18 L is provided to the guide thimble 2 where L is the length of the guide thimble 2 along its axial direction. Therefore, the compression load acting on the guide thimble 2 in the axial direction may cause flexural deformation of the dashpot 20. In this case, the control rod may not be inserted well.
For this reason, as shown in FIGS. 4 and 5, a technique is disclosed in which the length of the dashpot 20 of the guide thimble 2 is decreased. With this arrangement, the length of the dashpot 20 with respect to the length L of the guide thimble 2 can be suppressed to fall within the range of 0.03 L to 0.1 L, so the flexural rigidity of the dashpot 20 is increased. This can prevent flexural deformation of the dashpot 20.
This guide thimble will be referred to as an improved guide thimble hereinafter. The lower structure of a fuel assembly 1 to which an improved guide thimble shown in FIG. 5 is applied is different from that of the fuel assembly 1 shown in FIG. 2A only in that sleeves 21 are provided at the bottom grids 7 and that it has a dashpot 20 only at one portion, and is substantially the same as that of the fuel assembly shown in FIG. 2A.
In the fuel assembly to which the improved guide thimble is applied, the length of the dashpot 20 on the lower end side of the guide thimbles 2 is decreased, as shown in FIGS. 4 and 5. This increases the flexural rigidity of the dashpot 20 to prevent its flexural deformation. However, a so-called braking effect that moderates the fall velocity of the control rod is decreased.
In a PWR, its fall terminal velocity is limited from the viewpoint of ensuring the safety of the fuel assembly 1. Originally, the dashpot 20 is provided to the guide thimble 2 in an axial direction, as shown in FIG. 3, in order to moderate the fall velocity of the control rod such that the fall terminal velocity does not exceed a limit. For this reason, in the fuel assembly 1 employing the improved guide thimble as shown in FIGS. 4 and 5, a countermeasure that moderates the fall terminal velocity of the control rod must be provided by another means.
The present invention has been made in view of the above situation, and has as its object to provide a fuel assembly in which an improved guide thimble is employed and the diameter of the drain hole of a thimble screw is adjusted so that a fall impact produced when a control rod falls is moderated, and flexural deformation of a dashpot is prevented, without impairing the cooling function of non fuel bearing components, and a thimble screw of the fuel assembly.
In order to achieve the above object, the present invention has the following means.
According to a first aspect of the present invention, there is provided a fuel assembly comprising a bottom nozzle set on a lower core plate of a nuclear reactor, a top nozzle with a hold down spring to urge the bottom nozzle against the lower core plate, a plurality of control rod guide tubes which guide control rods, having passed through the top nozzle, toward the lower core plate, top, middle, and bottom grids mounted on the control rod guide tubes, a plurality of fuel rods held by the grids to be substantially parallel to the control rod guide tubes, a thin tube-like dashpot formed on each of the control rod guide tubes to reduce a fall velocity of a corresponding one of the control rods, a thimble screw which connects each of the control rod guide tubes to the bottom nozzle, and a drain hole formed to extend through the thimble screw. The dashpot has a large-diameter portion, at a lower portion thereof, with substantially the same diameter as that of each of the control rod guide tubes, and a diameter d of the drain hole falls within a range of 0.04 D less than d less than 0.08 D where D is an inner diameter of the large-diameter portion.
Hence, the coolant can be sufficiently supplied also from the viewpoint of assuring the cooling function of the non fuel bearing components. From the viewpoint of moderating the fall impact of the control rod as well, the terminal velocity of the control rod can be suppressed to be equal to or less than the fall velocity with which the fall impact of the control rod can be moderated. Therefore, flexural deformation of the dashpot can be prevented.
A fuel assembly according to a second aspect of the present invention has the following thimble screw.
More specifically, the thimble screw is disposed in a bottom nozzle so as to extend from a bottom surface side, has a drain hole extending in the thimble screw in a longitudinal direction from a spot facing hole of a seat to a distal end, is formed such that a coolant is supplied into the drain hole from the spot facing hole toward the distal end while the nuclear reactor operates and into the drain hole from a distal end side toward the spot facing hole during a scram mode, is locked to the bottom nozzle at the seat with a rotation preventive pin, and is provided with a coolant collision portion, at a drain hole side of the rotation preventive pin, against which the coolant flowing from the distal end side toward the spot facing hole collides in order to increase the pressure drop of the coolant during the scram mode.
Furthermore, a collision surface of this coolant collision portion against which the coolant collides forms a recessed surface ground in a V-shape from the distal end side toward the spot facing hole, a flat surface, or a recessed surface arcuately ground from the distal end side toward the spot facing hole.
With the above arrangement, the thimble screw can also serve as a diode. Thus, while the non fuel bearing components have the same cooling ability as that of the conventional case, the decelerating effect of the control rod can be improved by the increase in fluid resistance.
A fuel assembly according to a third aspect of the present invention has the following thimble screw.
More specifically, the thimble screw is disposed in a bottom nozzle to extend from a bottom surface side, has first and second drain holes extending in the thimble screw in a longitudinal direction from a spot facing hole of a seat to a distal end, and is formed such that a coolant is supplied into the first and second drain holes from the spot facing hole toward a distal end side while the nuclear reactor operates and into the second and first drain holes from the distal end side toward the spot facing hole during a scram mode. The thimble screws is locked to the bottom nozzle at the seat with a rotation preventive pin. The first drain hole has an opening area smaller than an opening area of the spot facing hole and smaller than an opening area of the second drain hole at the distal end.
Furthermore, the thimble screw is provided with a coolant collision portion, at a first drain hole side of the rotation preventive pin, against which the coolant flowing from the distal end side toward the spot facing hole collides in order to increase the pressure drop of the coolant during the scram mode.
The collision surface of the coolant collision portion against which the coolant collides forms a recessed surface ground in a V-shape from the distal end side toward the spot facing hole, a flat surface, or a recessed surface arcuately ground from the distal end side toward the spot facing hole.
Since the thimble screw has the above arrangement, the coolant enters from the distal end side and is discharged in the form of a jet from the drain hole toward the rotation preventive pin. As the rotation preventive pin strongly functions as a fluid resistance, the pressure drop for the flow of the coolant in the scram mode can be increased, and the decelerating effect of the control rod can be improved. Meanwhile, the rotation preventive pin does not influence the flow rate resistance of the coolant when the nuclear reactor operates. Thus, the coolant flow rate is assured, and the cooling ability of the non fuel bearing components can maintain the same effect as that of the conventional case.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.